This invention relates to devices which detect and/or treat tachyarrhythmias (rapid heart rhythms), and more specifically, to mechanisms to distinguish among various tachyarrhythmias and to provide appropriate therapies to treat the identified tachyarrhythmias.
Early automatic tachyarrhythmia detection systems for automatic cardioverter/defibrillators relied upon the presence or absence of electrical and mechanical heart activity (such as intra-myocardial pressure, blood pressure, impedance, stroke volume or heart movement) and/or the rate of the electrocardiogram to detect hemodynamically compromising ventricular tachycardia or fibrillation.
In pacemaker/cardioverter/defibrillators presently in commercial distribution or clinical evaluation, fibrillation is generally distinguished from ventricular tachycardia using ventricular rate based criteria. In such devices, it is common to specify the rate or interval ranges that characterize a tachyarrhythmia as opposed to fibrillation. However, some patients may suffer from ventricular tachycardia and ventricular fibrillation which have similar or overlapping rates, making it difficult to distinguish low rate fibrillation from high rate tachycardia. In addition, ventricular fibrillation may display Rxe2x80x94R intervals which vary considerably, resulting in intervals that may fall within both the tachycardia and fibrillation rate or interval ranges, or outside both. Similarly, supraventricular arrhythmias may be the cause of high ventricular rates, or may be present during ventricular arrhythmias, further increasing the possibilities of mis-diagnosis.
Presently available pacemaker/cardioverter/defibrillator arrhythmia control devices, such as the Model 7219 and Model 7217 devices commercially available from Medtronic, Inc., employ programmable fibrillation interval ranges and tachycardia detection interval ranges, along with measurement of suddenness of onset and rate variability. For future generations of devices, numerous detection and classification systems have been proposed. Numerous patents, including U.S. Pat. No. 5,217,021 issued to Steinhaus et al., U.S. Pat. No. 5,086,772 issued to Larnard et al., U.S. Pat. No. 5,058,599 issued to Andersen and U.S. Pat. No. 5,312,441 issued to Mader et al propose waveform morphology analysis systems for determining the type and origin of detected arrhythmias. Other patents, including U.S. Pat. No. 5,205,583 issued to Olson, U.S. Pat. No. 5,913,550 issued to Duffin, U.S. Pat. No. 5,193,535 issued to Bardy et al., U.S. Pat. No. 5,161,527 issued to Nappholz et al., U.S. Pat. No. 5,107,850 issued to Olive and U.S. Pat. No. 5,048,521, issued to Pless et al. propose systems for analysis of order and timing of atrial and ventricular events.
In the existing and proposed devices discussed above, one or two basic strategies are generally followed. A first strategy is to identify heart events, event intervals or event rates as they occur as indicative of the likelihood of the occurrence of specific types of arrhythmias, with each arrhythmia having a preset group of criteria which must be met as precedent to detection or classification. As events progress, the criteria for identifying the various arrhythmias are all monitored simultaneously, with the first set of criteria to be met resulting in detection and diagnosis of the arrhythmia. A second strategy is to define a set of criteria for events, event intervals and event rates which is generally indicative of a group of arrhythmias, and following those criteria being met, analyzing preceding or subsequent events to determine which specific arrhythmia is present. In the Medtronic Model 7219 devices, an arrhythmia detection and classification system generally as disclosed in U.S. Pat. No. 5,342,402, issued to Olson et al., incorporated herein by reference in its entirety, is employed, which uses both strategies together.
The arrhythmia detection and classification system of the present invention employs a prioritized set of inter-related rules for arrhythmia detection. Each rule contains a set of one or more xe2x80x9cclausesxe2x80x9d which must be satisfied (criteria which must be met). While all clauses of a rule are satisfied, the rule is indicated to be met. In the context of the present application this is referred to as the rule xe2x80x9cfiringxe2x80x9d. It is possible for multiple rules to be xe2x80x9cfiringxe2x80x9d at the same time, with the highest priority rule taking precedence. Some rules trigger delivery of therapy when firing. Other rules inhibit delivery of therapy when firing. The highest priority rule firing at any specific time controls the behavior of the device. For example, the firing of a rule which triggers therapy is superseded by the firing of higher priority rules preventing delivery of therapy. Rules cause firing when their clauses cease to be satisfied, whether or not a therapy is triggered by the rule.
Each rule includes a set of clauses which, when satisfied, indicate the likely occurrence of a specified type of heart rhythm, including various tachyarrhythmias, sinus tachycardia and normal sinus rhythm. A specific rhythm or tachyarrhythmia may have more than one associated rule. The rules are interrelated, such that progress toward meeting the requirements of a clause of one rule may also be the subject matter of a clause of a different rule.
The specific criteria set forth by the clauses of the various rules as disclosed include a number of known criteria for evaluating heart rhythm, including the entire arrhythmia detection and classification system employed in the presently available Medtronic 7219 pacemaker cardioverter defibrillators, as well as criteria disclosed in U.S. Pat. No. 5,330,508, issued to Gunderson, as will be discussed below. In addition, a number of new evaluation criteria are included within the clauses of various rules. One such new detection methodology is based upon the classification of the events occurring associated with the sequence of two ventricular depolarizations into a limited number of event patterns, based upon the number and times of occurrences of atrial events, preceding the two most recent ventricular events. An event pattern is developed for each individual ventricular event, so that successive event patterns overlap one another. The inventors have determined that certain sequences of event patterns are strongly indicative of specific types of heart rhythms. For heart rhythms of which this is true, a defined set of indicative event pattern sequences or a xe2x80x9cgrammarxe2x80x9d is defined. Adherence of the heart rhythm to the grammars associated with various heart rhythms is determined by simultaneously operating continuous recognition machines, the outputs of which form the subject matter of one or more clauses, within the hierarchy of rules.
An additional new classification criteria included in some rules is a measure of the co-variance of the R wave to P wave interval with the R wave to R wave interval, which is employed as an index of the likelihood that the depolarizations of the ventricles over a preceding series of events have had an atrial origin. This criterion is applied in conjunction with rules intended to identify the likely occurrence of atrial fibrillation and atrial flutter.
An additional new classification criterion comprises a methodology of identifying the likelihood that events sensed in the atrium are in fact far field R waves, rather than P waves. The occurrence of such identified far field R waves over a sequence of Rxe2x80x94R intervals is employed in conjunction with rules indicative of various types of supraventricular tachycardias.